5.1
Tropical Atmospheric Forcing of the Wintertime North Atlantic Oscillation

The relationship between the interannual wintertime variability of the North Atlantic Oscillation (NAO) and tropical heating anomalies is examined using the NCEP-NCAR reanalysis and observation based sea surface temperature (SST) and precipitation data for the period from 1980-2011. The NAO is found to be significantly correlated with the precipitation anomalies over the tropical Indian Ocean and tropical America-Atlantic, but not with the underlying SST anomalies. The anomalous tropical Indian Ocean heating is related to the Madden-Julian Oscillation (MJO) occurrence in that region.

The tropical heating impact on the NAO is examined and the evolution process of the influence is explored by numerical experiments using a primitive equation atmospheric model forced by atmospheric heating perturbations. Results from both the reanalysis data and numerical experiments suggest that the tropical Indian Ocean atmospheric heating appears to be a driving forcing for the NAO variability. The atmospheric response to the tropical forcing involves the combined effects of Rossby wave dispersion, normal mode instability and transient eddy feedback. The influence of the forcing on the NAO tends to be organized and achieved by the circumglobal teleconnection pattern. By contrast, the influence of the tropical American-Atlantic heating on the NAO appears to be weak. The linkage between the NAO and the tropical American-Atlantic heating is likely through the anomalously meridional atmospheric circulation over the Atlantic Ocean.